[release-branch.go1.20] cmd/compile/internal/noder: stop creating TUNION types

In the types1 universe under the unified frontend, we never need to
worry about type parameter constraints, so we only see pure
interfaces. However, we might still see interfaces that contain union
types, because of interfaces like "interface{ any | int }" (equivalent
to just "any").

We can handle these without needing to actually represent type unions
within types1 by simply mapping any union to "any".

Fixes #58413.

Change-Id: I5e4efcf0339edbb01f4035c54fb6fb1f9ddc0c65
Reviewed-on: https://go-review.googlesource.com/c/go/+/458619
Run-TryBot: Matthew Dempsky <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
(cherry picked from commit a7de684)
Reviewed-on: https://go-review.googlesource.com/c/go/+/466435
Reviewed-by: Than McIntosh <[email protected]>
Run-TryBot: David Chase <[email protected]>

Author: mdempsky

src/cmd/compile/internal/noder/reader.go

@@ -517,13 +517,33 @@ func (r *reader) doTyp() *types.Type {
 }
 
 func (r *reader) unionType() *types.Type {
-	terms := make([]*types.Type, r.Len())
-	tildes := make([]bool, len(terms))
-	for i := range terms {
-		tildes[i] = r.Bool()
-		terms[i] = r.typ()
+	// In the types1 universe, we only need to handle value types.
+	// Impure interfaces (i.e., interfaces with non-trivial type sets
+	// like "int | string") can only appear as type parameter bounds,
+	// and this is enforced by the types2 type checker.
+	//
+	// However, type unions can still appear in pure interfaces if the
+	// type union is equivalent to "any". E.g., typeparam/issue52124.go
+	// declares variables with the type "interface { any | int }".
+	//
+	// To avoid needing to represent type unions in types1 (since we
+	// don't have any uses for that today anyway), we simply fold them
+	// to "any". As a consistency check, we still read the union terms
+	// to make sure this substitution is safe.
+
+	pure := false
+	for i, n := 0, r.Len(); i < n; i++ {
+		_ = r.Bool() // tilde
+		term := r.typ()
+		if term.IsEmptyInterface() {
+			pure = true
+		}
+	}
+	if !pure {
+		base.Fatalf("impure type set used in value type")
 	}
-	return types.NewUnion(terms, tildes)
+
+	return types.Types[types.TINTER]
 }
 
 func (r *reader) interfaceType() *types.Type {

test/typeparam/issue52124.go

@@ -6,7 +6,9 @@
 
 package p
 
-type I interface{ any | int }
+type Any any
+
+type I interface{ Any | int }
 
 var (
 	X I = 42